Yep, that’s about what I came up with, as well. There are online “pond volume calculators,” but the most accurate way to measure is with a water gauge when filling the pond.
I got almost 700 gallons on the pond calculator the first time, too. But I reduced the depth to an average of 18" and got 585 gallons. Again, just an estimate. As @Mmathis said, the only way to know for sure is to use a meter when you fill.
I know sometimes it's not practical to use a meter if the pond is filled already or if was partially filled when building it and then filled again when all the rocks, waterfalls, etc are installed. I visited someone over the summer on a pond tour who added salt I think to his pond and used a device that that measured the concentration. The density of the added compound gave him the volume. It came in at 10,800 gallons!
I like to see how much the water level goes up when I add water. Right now my pond goes up an inch for every 90 gallons I add. I know that's imperfect because my pond does not go straight down on the sides but it is pretty close to that. So if my pond is 36 inches deep on average x 90 gallons per inch gives me a rough idea of the size. So even if I take a little off and say 80 gallons per inch it gets me very close to the actual volume of my pond. (2800-3000 gallons)
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